Polyphenylene ethers have been used widely as a material for products or parts in electrical or electronic fields, automotive fields, other industrial material fields, and food or packaging fields, because they are excellent in workability and productivity, and can be produced as products or parts of a desired shape by melt injection or melt extrusion molding methods.
A number of preparation processes of polyphenylene ethers have been proposed, for example, by JP-B-36-18692 (The term “JP-B” used herein means an “examined Japanese patent publication”) and U.S. Pat. Nos. 3,306,875, 3,344,116, and 3,432,466.
The polyphenylene ether, which is obtained by said known method and used in the above-mentioned fields, is a high molecular weight polymer having a reduced viscosity of not less than 0.3 dl/g. Nowadays, however, it is expected that a very low molecular weight polyphenylene ether is more suitable for an electronic material such as a printed circuit board than above-mentioned high molecular weight polyphenylene ether. As the electronic material such as a printed circuit board has become more sophisticated recently, the conductivity-loss of a circuit itself has been more noticeable. Such a conductivity-loss is emitted as heat. Therefore, the material of a printed circuit board is required to have not only good dielectricity but also high thermal resistance. In the process for producing a printed circuit board, it is common to dissolve a material into a solvent to functionalize it by a modification reaction, and conduct a reaction such as thermosetting. Accordingly, it is necessary that a material of a printed circuit board is excellent in solubility in a solvent and have good reactivity with other components.
A low molecular weight polyphenylene ether is disclosed in JP-B-50-6520, JP-A-62-39628 (The term “JP-A” used herein means an “unexamined Japanese patent publication”) and U.S. Pat. No. 6,211,327. However, no low molecular weight polyphenylene ether obtained by the methods disclosed in the above-mentioned documents is satisfactory in the point of thermal resistance and electric properties.
JP-B-50-6520 describes a-process for producing polyphenylene ether in a mixed solvent comprising an aromatic hydrocarbon solvent such as benzene, toluene and xylene, and an aliphatic hydrocarbon solvent such as n-hexane, isohexane and n-heptane. However, the resultant low molecular weight polyphenylene ether obtained by the method is not satisfactory in the point of thermal resistance and electric properties. As is apparent from the description in the example, when the reduced viscosity of the resultant polyphenylene ether is not more than 0.2 dl/g, the polyphenylene ether adheres on a reactor or the like to reduce the yield.
When a good solvent of polyphenylene ether (an aromatic hydrocarbon solvent such as benzene, toluene and xylene in the above-mentioned document) is, used as a polymerization solvent, the yield is reduced by adhesion of a low molecular weight polyphenylene ether on a reactor since a good solvent has good affinity for the resultant low molecular weight polyphenylene ether. In addition, problems occur such that facilities to remove a good solvent from polyphenylene ether become excessive and that the temperature of such facilities must be controlled very carefully. Therefore, a reduced viscosity at which the low molecular weight polyphenylene ether can be obtained efficiently has remained around 0.2 dl/g at most.
JP-A-62-39628 teaches that, in a process for producing low molecular weight polyphenylene ether having a number average molecular weight smaller than 2,800, a monofunctional alcohol having 1-5 carbon atoms is used optionally with water as a polymerization solvent. The low molecular weight polyphenylene ether obtained by the process cannot be said to have sufficient thermal resistance and electric properties. Moreover, the process described in the document has problems that the polymerization yield achieved is only 95% at maximum, in the examples less than 90%, and that the polymerization takes an extremely long time as described in the examples. Therefore, the process is not efficient in industry.
U.S. Pat. No. 6,211,327 describes a process for producing a low molecular weight polyphenylene ether, which comprises extracting catalyst components from a polyphenylene ether solution in a water phase and then devolatilizing a good solvent of polyphenylene ether from the polyphenylene ether solution using, for instance, a devolatilization extruder. This process is an improved method for producing very low molecular weight polyphenylene ether having a reduced viscosity of 0.1 dl/g or less, which process is free from problems of yield. The low molecular weight polyphenylene ether obtained by this method, however, cannot be said to have sufficient thermal resistance and electric properties.
Further, the low molecular weight polyphenylene ether obtained by the above method is generally in the form of granular particles or pellets. Because large sized particles are contained in the polyphenylene ether, troubles arise such that it requires a long time to dissolve in a solvent or adhere on a reactor in a half-dissolved state.